I’m accustomed to ____ water.

A. Drink
B. Drinking
C. Drank
D. Drunk

I’m accustomed to Drinking  water.

Here, drinking is the right answer as accustomed to is followed by the gerund. Here, gerund i.e. verb+ing is drinking.

Accustomed to

Accustomed to is an adjective that means used to with something. It means in the habit of or adapted to something.

Example

I’ve grown accustomed to her glamorous face.

Accustomed to Synonyms

The adjective accustomed to has the following synonyms

Adjusted to, in the habit of, familiar with, acclimatized to, given to, prone to, practised in
Formal habituated to, inured to, wont to
Colloq. at home with, no stranger to

What is Gerund?

Ans: A verb form that functions as a noun and ends with -ing

Example

“Playing” in the following sentence
Playing badminton in the winter is not easy?